The present invention relates to a novel method of detecting low levels of impurities in lubricants for compressors. More particularly, this invention relates to a novel but simple method for detecting low levels of alklybenzene or mineral oil which may be present as an impurity in polyol ester (POE) lubricants in compressors.
Lubricants are generally added in refrigeration units in order to protect and prolong the life of the frictionally engaged machine components from wear. One class of lubricants comprises, for example, mineral oils. They are produced by known techniques such as by distillation of crude and subsequent refining of petroleum. Depending on the origin of the raw material petroleum, mineral oils produced may be designated paraffin-or naphthenic-base lubricating oil. Paraffin-base lubricating oils comprise paraffinic molecules while naphthenic-base oil comprise napthenic or cycloparaffinic molecules. These oils generally do not contain additives. To improve wear resistance additives such as phosphate esters have been suggested.
Another class of lubricants comprises alkylbenzenes. They have the general formula as shown in FIG. 1. ##STR1## in which n is a positive integer.
The alkylbenzenes are produced by the known Friedel Crafts synthesis such as by alkylation of benzene with alkylchlorides or olefins. Lubricants serve a very important function in refrigeration units. Low levels of lubricants or impurities therein could cause premature fatigue resulting in failure of engaged machine components. A satisfactory lubricant should exhibit good low-temperature characteristics and thermal stability.
Commercial manufactures such as Thermo King has a large number of refrigeration units in the field which are operating with R-12 which is a fluoro carbon refrigerant and alkylbenzene or mineral oil as lubricants in the refrigeration systems. With the legislative changes which prohibit the use of high Ozone Depletion Potential (ODP) and high Global Warming Potential (GWP) refrigerants, R-I2 refrigerant will be banned in the near future. The retrofit of these compressors will require a change from chlorinated fluorinated hydrocarbon (CFCs) to fluorinated hydrocarbon (HFCs), such as R-134a. Due to incompatibility of R-134a which is tetrafluoro ethane, with the presently used lubricants (alkylbenzene and mineral oil), new polyol ester (POE) lubricants have to be used. The retrofit procedure recommended by the refrigerant and lubricant manufacturers and our own studies suggest at least three separate lubricant changes in the compressor in order to remove the existing lubricant to an acceptable level.
It has been documented in some studies and further confirmed by miscibility studies that even low concentrations of alkylbenzene or mineral oil in polyol ester can alter the low temperature miscibility drastically. Poor miscibility will affect the performance of the lubricant at low temperature. Table 1 shows the adverse effects of low levels of mineral oil which is present as an impurity in the Castrol SW-32 a polyol ester lubricant.
TABLE 1 ______________________________________ Effect of Mineral Oil Impurity in POE Lubricant on Miscibility Miscibility of 90% R-134a and 10 SW-32 Mineral Oil Lowest Impurity Level Temperature (% of Oil) Miscibility .degree.C. ______________________________________ 0 &lt;-50 1 -35 3 0 5 +18 ______________________________________
These results suggest that it is important to reduce the mineral oil/alkylbenzene concentration in POE to a minimum during retrofitting.
At present there is no practical method to detect low concentrations of impurities in the form of alkylbenzene/mineral oil in POE, especially where the refrigeration units had been installed. Typical procedures recommended to refrigeration engineers for retrofitting requires three changes of the POE lubricant, which reduces the concentration of alkylbenzene or mineral oil to less than 3%. The method used for testing the presence of the impurity involves withdrawing a sample and then determining the level of the said impurity by infra-red (IR) spectroscopy. When an operator is carrying out this procedure in the field, it is obviously inconvenient to service the test and requires an IR instrument.